This application is based on Japanese Patent Applications No. 2001-309846 filed on Oct. 5, 2001 and No. 2002-202579 filed on Jul. 11, 2002 the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a control system for an internal combustion engine (engine).
2. Description of Related Art
It is preferable to decrease a maximum combustion temperature of fuel in a combustion chamber in order to reduce emissions of nitrogen oxides (NOx) and particulates. Conventionally, increasing an amount of exhaust-gas recirculation, or delaying fuel injection is carried out to decrease the temperature in the combustion chamber. However, the temperature in the combustion chamber is still increased in a high engine load condition according to the conventional measures.
JP-A-11-315739 discloses a combination of low compression ratio engine and an engine control apparatus. According to JP-A-11-315739, the engine having a relatively low theoretical compression ratio can decrease a compression end temperature. The theoretical compression ratio is determined geometrically based on the engine dimensions. The compression end temperature is a temperature in the combustion chamber just before beginning combustion.
However, the temperature in the combustion chamber is originally low during the engine starting and a low engine load condition. Therefore, decreasing the theoretical compression ratio may cause an instable ignition of fuel and make it difficult to operate the engine stably. In order to avoid such disadvantages, JP-A-11-315739 discloses the engine control apparatus that advances a fuel injection timing and advances closing timing of intake valves so that the compression end temperature is maintained above a predetermined temperature. Advanced closing of the intake valves increases an actual compression ratio since it decreases a back flow of intake air. The actual compression ratio may be referred to as an effective compression ratio. Advanced fuel injection timing allows a fuel injection when a compressed air in the combustion chamber still keeps a high temperature, and provides a relatively stable ignition and combustion. According to the JP-A-11-315739, it is possible to reduce the emissions in a medium engine load and to keep combustions stable in a low engine load.
In case of an internal combustion engine for a vehicle, the engine is regularly used under the low engine load condition such as a driving in a city. In such a low engine load conditions, the advanced closing of the intake valve or the advanced fuel injection may cause an excess increase of combustion temperature and cause an increase of the emissions.
It is an object of the present invention to provide a control system for an engine that is capable of operating the engine stably and reducing emissions of NOx and particulates.
It is another object of the present invention to operating the engine stably and reducing the emissions without decreasing the theoretical compression ratio.
According to an embodiment of the present invention, it is possible to decrease temperature in a combustion chamber of the engine. During the high engine load, heat generated in the combustion chamber is increased due to an increase of fuel injection amount. When the engine is operated under a high engine load, an actual compression ratio is decreased. For example, a closing timing of an intake valve of the engine is delayed. However, decreasing the actual compression ratio may cause a decrease of heat capacity in the combustion chamber, and cause an increase of a maximum combustion temperature. In order to avoid such disadvantage, an amount of intake air induced into the combustion chamber is increased. For example, a boost pressure of a forced induction system is increased. The increased boost pressure is set higher than a boost pressure set before the actual compression ratio is decreased. As a result, it is possible to prevent decreasing of the heat capacity, and to decrease the maximum combustion temperature. Therefore, it is possible to reduce the emissions of NOx and particulates. Further, such advantages can be achieved without decreasing a theoretical compression ratio.
Further, an EGR cooler may be controlled in accordance with the engine load. For example, the EGR cooler is deactivated when the engine load is in a low engine load range in order to increase temperature of the intake air. Therefore, it is possible to improve stability of the engine in the low engine load. The EGR cooler may be deactivated when the engine is in a warming up operation.
Further, the present invention is effective for an engine that is operated under a relatively large amount of EGR and a relatively delayed injection timing. It is possible to enhance reduction of the emissions of NOx and particulates, and to provide a stable operation of the engine.